The present invention relates to a method of producing an electroluminescence (hereinafter referred to as "EL") emitting film containing sulfide of Sr, Ca, Zn, etc. as a base material and a rare earth element such as Ce, Eu, Pt, etc. as an emitting center and being adapted to integration into a thin-film lamination structure type EL display panel or the like.
As well known, an EL display panel has characteristic of self emission. Particularly, the EL display panel of a thin-film lamination type can display a variable image not only highly densely on a large-sized screen but sharply at a high luminance. Use of this type EL display panel as a display unit in the field of Office Automation apparatus, small-sized computers, etc. has been widened rapidly. In this type EL display panel which has been put into practical use, in most cases, used is an EL emitting film which contains zinc sulfide as a base material and a slight amount of manganese as an emitting center and which emits a yellowish green light. Though known well, the outline of the structure of this type EL display panel will be described briefly with reference to FIG. 6.
FIG. 6 is an enlarged sectional view of a peripheral portion of a thin-film lamination structure type EL display panel 10. A plurality of front-surface electrode films 12 made of transparent electrically conductive films are provided on a surface of a substrate 11 made from transparent glass so that they extend back and forth in this drawing and are arranged left and right in this drawing so as to be shaped like a pattern of stripes. After the front-surface electrode films 12 are covered with an insulating film 13, an EL emitting film 14 containing manganese-including zinc sulfide, etc. is produced. After the EL emitting film 14 is covered with an insulating film 15, a plurality of rear-surface electrode films 16 made from aluminum are provided so that they extend left and right and are arranged back and forth so as to be shaped like a pattern of stripes. Display voltages switched between positive values and negative values by one frame period are supplied between the front-surface electrode films 12 and the rear-surface electrode films 16 from a display drive circuit 1 to thereby perform display on the display panel 10. As a result, portions of the emitting film 14 constituted by intersection points of the two kinds of electrode films 12 and 16 perpendicular to each other perform EL emission as pixels respectively. Accordingly, the EL emission light is taken out as display light DL from the transparent front-surface electrode film side.
In the case where the emitting film 14 only contains the above-mentioned manganese-including zinc sulfide, it is a matter of course that color display cannot be performed because the emission light color is limited. For this reason, there are attempts to produce EL emitting films which use sulfides of strontium Sr, calcium Ca, etc. as a base material and are added with rare earth elements such as cerium Ce, europium Eu, praseodymium Pr, etc. as an emitting center to thereby exhibit various emission light colors. As methods of producing such films, there are mainly known an electron beam evaporation method, a CVD method and a sputtering method.
As well known, in the electron beam evaporation method, an electron beam is applied to an evaporation source prepared by adding an emitting center element to a base material to heat the evaporation source locally to thereby evaporate the evaporation source rapidly. In the CVD method, a gasified compound containing a base material element and an emitting center element as a raw material gas is decomposed in an atmosphere of plasma or the like. In the sputtering method, a surface portion of a base material containing an emitting center element as a target is hit with a sputtering gas to thereby make the target fly. As a result, the evaporated, decomposed or flying matter is accumulated as an EL emitting film.
When EL emitting films having various emission colors are to be produced, the above-mentioned methods have respective problems.
The electron beam evaporation method has performance upon production of the above-mentioned emitting film of zinc sulfide including manganese added thereto and has an advantage in that the chemical composition of the emitting film is exact so that the distribution of the emitting center element in the film is uniform. When the electron beam evaporation method is however applied to production of the EL emitting film containing, as a base material, sulfide of Sr or Ca added with a rare earth element, the evaporation source heated rapidly is apt to be evaporated in the form of massive particles and deposited in this form. As a result, particles with the size of several microns [.mu.m] are mixed into the emitting film so that the surface of the emitting film becomes greatly rough. There is a tendency that breakdown of the insulation film being in contact with the projecting portion of the emitting film is caused by concentration of electric field to the projecting portion of the emitting film when display voltages are supplied. Further, in this method, the point of evaporation of the evaporation source is limited to a local portion which can be heated by means of an electron beam. Accordingly, in the case of a large-area display panel with the diagonal size of 20 inches or larger, it is very difficult to produce the emitting film uniform in thickness on the surface of the display panel.
In the CVD method, there is no problem in mixing of large particles into the emitting film. In the CVD method, there is however the largest problem that the film-producing speed is so low that the method cannot be adapted to mass production. Further, it is practically difficult to produce a large-area emitting film uniform in film quality by this method.
On the contrary, in the sputtering method, the material is made to fly from the whole surface of the target in the form of fine particles or molecules and deposited on the emitting film. As a result, there is no mixing of large particles into the emitting film, so that a large-area film uniform in film thickness and film quality can be produced. Accordingly, the film-producing speed is so high that this method can be adapted to mass production. It is therefore thought of that the sputtering method is a most promising method for production of emitting films exhibiting various emission light colors.
In the conventional sputtering method, it is however difficult to produce a high-luminance emitting film because shortage of sulfur at the surface portion of the target is caused by the high vapor pressure of sulfur contained in sulfide of the base material so that the base material on which the emitting film is produced is apt to be a sulfur-defective composition departing from the stoichiometric and accordingly deteriorated in crystalline characteristic. It is further difficult to control the rare earth element concentration of the emitting film, because the sputtering rate of the base material is different from that of the rare earth element so that the rare earth element concentration of the emitting film becomes different from that of the target.